Add requirements to categories

[ibboard]

Snowblizz has thought up a new requirement type that we need to add to the list: limits on groupings or categories.

The example is GW's Battlefleet Gothic. In it you can have 1 Battleship for every 3 cruisers, 1 grand cruiser for every 3 cruisers and 1 heavy cruiser for every 2 cruisers (for example). A "Battleship" isn't a specific class of ship (unit type), though, but is a grouping of similar sized ships.

For more conventional games, a similar limitation might be "1 cavalry for every 3 infantry".

At its simplest level, this could just be a category (e.g. "1 hero for every 3 troops") but ideally we'd eventually implement an arbitrary grouping option.